Teeth are weakened over their life-span by decay, recurrent decay, root canal accesss, loss of blood supply, excessively large fillings and heavy wear. Weakened teeth will usually, given enough time, split, break or fracture. These splits, breaks or fractures can be a death sentence for the tooth. When a fracture does occur, sometimes a crown can save that tooth--but in actuality, the proper thing to do is restore a compromised tooth BEFORE it breaks.
The tooth to the right, with a large filling and recurrent decay, has split into two halves. It is now unrestorable and must be extracted.
During examinations, your dentist will point out weak, split, cracked or decayed teeth which they believe to have a high possibility for damage. A crown may be suggested to protect these type of compromised teeth.
Unlike a car, which we can run until it breaks down and then fix it up with new parts. Often a tooth cannot be fixed and there are no "teeth parts" you can install when one is lost (at least not real teeth). As such, taking definitive, proper, preventive steps to keep splits, fractures or breaks from happening only makes sense.
Large fillings, like the one to the right, very often have thin portions of tooth on either side of the restorative material. This thin tooth structure is intrinsically weak and even moreso when all of the inner layer--the dentin--is removed from under the enamel. This is called unsupported enamel and it is usually just a matter of time before it breaks. If the break is too low on the tooth, it can mean expensive crown lengthening surgery to save it or it may even mean that its non-restorable and will need to be extracted.
A crown is fabricated by a dental laboratory. Once completed, it is fit over the prepared tooth and it's fixed in place with a strong dental cement.
This type of restoration protects weak, compromised teeth from fracture and splitting.
The photo to the left shows a cut away portion of an all porcelain crown. Crowns, covering the thin, weak walls and cusps of compromised teeth, help to hold the tooth together and prevents tooth fracture. Forces from opposing teeth cannot be directed on the weak cusp any longer. Before the crown, great force could be directed right on the weak, thin enamel cusp. Now the force is directed over the whole tooth because the crown is inflexible.
Everyday, dentists see broken teeth in the mouths of their patients. These are most often teeth that have been compromised severly with large fillings, heavy decay or those teeth with large openings in them, like those left behind after a root canal procedure.
Attempting to restore a tooth with a large filling, when all the criteria dictate that a crown is needed, can ultimately result in tooth loss. It also puts the attending dentist in a situation where he is outside of the standard of care. This is a concept used by courts and state dental boards to determine whether or not a professional has acted appropriately in relationship to rendered care
With some teeth, failure to crown the tooth when cracks appear, can result in a tooth that splits completely and its not restorable. Such is the case with the worn incisor tooth to the left.